(Stroke. 2000;31:420.)
© 2000 American Heart Association, Inc.
Original Contributions |
From the Departments of Epidemiology (O.H.K., R.C.K., B.M.P., S.R.H., W.T.L.), Medicine (B.M.P., N.L.S., R.N.L., W.T.L.), Health Services (B.M.P.), and Neurology (W.T.L.), Cardiovascular Health Research Unit, University of Washington, Seattle; and the Department of Pharmacoepidemiology and Pharmacotherapy (O.H.K., H.G.M.L., A. de B.), Utrecht Institute of Pharmaceutical Sciences, Utrecht University (Netherlands).
Correspondence to Olaf H. Klungel, Department of Pharmacoepidemiology and Pharmacotherapy, Utrecht Institute of Pharmaceutical Sciences, Utrecht University, PO Box 80082, 3508 TB, Utrecht, Netherlands. E-mail o.h.klungel{at}pharm.uu.nl
| Abstract |
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MethodsA population-based case-control study was conducted among treated hypertensive members of Group Health Cooperative of Puget Sound. Cases were treated hypertensive patients who sustained a first fatal or nonfatal, ischemic (n=460) or hemorrhagic (n=95) stroke during 19891996. Controls were a random sample of stroke-free, treated hypertensive Group Health Cooperative enrollees (n=2966), similar in age to the stroke cases. Multiple measurements of blood pressure and other cardiovascular risk factors were collected from medical records. Logistic regression was used to estimate the risk of ischemic stroke and hemorrhagic stroke associated with uncontrolled blood pressure, defined as diastolic blood pressure >90 mm Hg or systolic blood pressure >140 mm Hg. The fraction of strokes attributable to uncontrolled blood pressure among treated hypertensives was calculated.
ResultsBlood pressure was uncontrolled in 78% of ischemic stroke cases, 85% of hemorrhagic stroke cases, and 65% of controls. After adjustment for potential confounders, uncontrolled blood pressure among treated hypertensive patients was moderately associated with ischemic stroke (risk ratio=1.5 [95% CI, 1.2 to 1.9]) and strongly related to hemorrhagic stroke (risk ratio=3.0 [95% CI, 1.7 to 5.4]). We estimated that 27% (95% CI, 11% to 39%) of the ischemic strokes and 57% (95% CI, 26% to 75%) of the hemorrhagic strokes among treated hypertensive patients were attributable to uncontrolled blood pressure. Overall, 32% (95% CI, 14% to 45%) of all strokes were attributable to uncontrolled blood pressure.
ConclusionsA considerable proportion of incident strokes among treated hypertensive patients may be prevented by achieving control of blood pressure.
Key Words: hypertension pharmacology risk stroke, hemorrhagic stroke, ischemic
| Introduction |
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| Subjects and Methods |
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We used data from a population-based case-control study that was originally conducted to study the associations between antihypertensive therapies and myocardial infarction and stroke. Our methods were similar to those used in previous case-control studies.7
Subjects
Cases were GHC enrollees, aged 30 to 79 years, who were treated
pharmacologically for hypertension and who sustained an incident fatal
or nonfatal stroke during July 1989 through December 1996. Potential
cases were identified from computerized GHC hospital discharge
abstracts, Washington state death files, and the billing records
for GHC enrollees who received medical care or services from non-GHC
providers. Controls were obtained from a companion study of risk
factors for myocardial infarction at GHC.7 Controls were a
random sample of GHC enrollees who were treated pharmacologically for
hypertension, frequency matched to the myocardial infarction cases by
sex, age, and calendar year. Each subject was assigned an index date.
For the cases, the index date was the date of the stroke; for controls,
the index date was a random date within the calendar year for which
they had been selected as controls. In most strata based on sex,
10-year age categories, and index year, the ratio of controls to cases
was >3:1. We excluded subjects (1) who were enrollees for <1 year or
who had <4 visits before their index dates; (2) who had had a prior
stroke; (3) who had a diagnosis of congestive heart failure; (4) whose
stroke was a complication of a procedure or surgery; and (5) who had no
recorded BP in their medical records within the year before the
index date.
Data Collection and Definitions
Information on BP and other cardiovascular
disease risk factors was abstracted from medical records and
obtained from a telephone interview of consenting survivors.
Abstraction of the information from the medical records was done by
trained research assistants who were not blinded to case-control status
but were unaware of the study hypothesis. Subjects were considered
pharmacologically treated for hypertension when a recording of
antihypertensive drug use for the indication of hypertension was
present in the medical records and indicated that the subject
was treated at the index date. Control of BP was defined according to
the mean of the last 3 treated BP readings recorded in the medical
records during the year before the index date. If <3 readings were
available, we used either the mean of 2 readings (n=563) or a single
reading (n=473) to assess the level of BP achieved by treatment.
The level of BP achieved by treatment was categorized according to the classification scheme of hypertension of the sixth report of the Joint National Committee (JNC-VI) on the Prevention, Detection, Evaluation, and Treatment of High Blood Pressure.4 According to this classification, a diastolic blood pressure (DBP) <90 mm Hg and a systolic blood pressure (SBP) <140 mm Hg are considered controlled. This classification would provide the most informative results for clinicians and policy makers in the United States to assess the implications of managing hypertension according to these national practice guidelines.
Statistical Analysis
Complete data were uniformly available from the medical
records for case-control status and medical conditions such as
pharmacological treatment of hypertension, angina, and diabetes.
In preliminary analyses of demographic and behavioral risk factor data such as smoking, physical activity, race, marital status, and education, the agreement between medical record and self-reported measures was good to excellent. Self-reported data, available for 52.4% of the subjects, were used for these variables; if not available, then data from the medical record were used. After information from these 2 sources was combined, data were missing on smoking (1.0%), physical activity (8.0%), race (2.4%), education (28.1%), cholesterol (5.0%), duration of treatment for hypertension (10.2%), and pretreatment BP (29.7%). We used an approximate Bayesian bootstrap method to impute these missing values. This multiple imputation method is a modification of the hot-deck method and takes account of the imputation variability.8 In sensitivity analyses, the results using multiply-imputed data were similar to those seen in the analysis of subjects with complete data.
All statistical tests were 2-tailed. We used stratification and
logistic regression to control for potential confounding factors of the
association between BP control and stroke. We used odds ratios to
estimate the risk of stroke associated with the various JNC-VI stages
of hypertension,4 compared with the risk in subjects with
controlled BP (DBP <90 mm Hg and SBP <140 mm Hg). The
population attributable fractions and their 95% CIs were calculated
according to Whittemore.9 We also examined the relation
between tertile of DBP (bottom tertile, DBP <82 mm Hg; middle
tertile, DBP 82 to 90 mm Hg; top tertile, DBP >90 mm Hg)
and SBP (bottom tertile, SBP <146 mm Hg; middle tertile, SBP 146
to 159 mm Hg; top tertile, SBP
159 mm Hg) and risk of
stroke. All analyses were performed separately for
ischemic and hemorrhagic strokes.
| Results |
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Compared with controls, ischemic stroke cases had higher
mean levels of treated SBP, pretreatment SBP, and total
cholesterol (Table 1
). A
number of risk factors, including diabetes and history of
cardiovascular disease, were more prevalent among
ischemic stroke cases than among controls. Compared with
controls, hemorrhagic stroke cases had higher mean levels of treated
SBP and DBP, were more often current smokers, and more often had a
history of transient ischemic attack, but not of
cardiovascular disease (Table 1
).
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The mean BP during the year before the index date was more often
uncontrolled among ischemic (77.6%) and hemorrhagic (85.3%)
stroke cases than among controls (65.3%) (Table 2
). Uncontrolled BP was moderately
associated with ischemic stroke (risk ratio [RR]=1.52 [95%
CI, 1.19 to 1.94]) and strongly related to hemorrhagic stroke (RR=3.03
[95% CI, 1.69 to 5.41]). The relative risks progressively increased
with the level of uncontrolled BP defined according to the JNC-VI
classification of hypertension, more so for hemorrhagic stroke than for
ischemic stroke (Table 2
).
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The association between both types of stroke and uncontrolled BP did not significantly differ between men and women, between older and younger subjects (above and below the median age), or between subjects with or without a history of cardiovascular disease, with or without diabetes, or with high or low pretreatment DBP or SBP (above and below the median pretreatment BP). The results remained virtually unchanged after adjustment for pretreatment DBP, duration of treated hypertension, body mass index, educational level, marital status, race, physical activity, and current use of aspirin. Exclusion of subjects who had <3 BP readings in their medical records before the index date did not substantially change the results.
Estimation of the population attributable fractions suggests that 27% (95% CI, 11% to 39%) of ischemic and 57% (95% CI, 26% to 75%) of hemorrhagic strokes may have been attributable to uncontrolled BP. One third of the ischemic strokes and one half of the hemorrhagic strokes that were attributable to uncontrolled BP occurred among those with mildly elevated BP levels (JNC-VI stage I). Overall, 32% (95% CI, 14% to 45%) of all strokes may have been attributable to uncontrolled BP.
The association between ischemic stroke risk and tertile
of BP was similar for DBP and SBP, whereas the risk of hemorrhagic
stroke was more strongly associated with SBP than with DBP (Table 3
).
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| Discussion |
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The strengths of this observational study include the use of population-based case and control subjects, the comparable ascertainment of potential confounding factors, and the use of multiple measurements of BP to assess control of BP. Although we adjusted for potential confounding factors, residual confounding cannot be excluded because of the possibility of incomplete or inaccurate measurement of covariates and unknown or unmeasured confounding factors for which adjustment was not possible. Furthermore, the medical records abstractors were not blinded to case-control status and may have classified potential confounding factors differently for cases and controls. In approximately 30% of subjects there were no data on pretreatment BP values. Although the confounding effects of pretreatment BP levels were virtually the same in the analysis of subjects with complete data as in the analysis of subjects with missing pretreatment BP values, we cannot completely rule out the possibility that subjects with uncontrolled BP had higher pretreatment BP levels and therefore a higher risk of stroke.
While stroke risk is clearly reduced by BP treatment in clinical trials,10 11 12 the population attributable fractions that we estimated may not directly apply to other settings in which population characteristics and levels of BP control differ. The general US population includes more blacks, Hispanics, and uninsured people who are more likely to have poor BP control,1 2 13 14 and this difference may limit the generalizability of our findings. The rate of control of hypertension in the United States according to the NHANES-III survey (19881991)2 was, however, higher (45%) than the rate of control of hypertension among the controls in our study population (34.7%). This difference may have been due to the smaller number of BP measurements in our study (3 during 1 year) compared with the NHANES-III survey (6 during 2 visits), which would tend to overestimate the prevalence of hypertension.14a
This study not only confirms the strong association between achieved BP and risk of stroke among treated hypertensive patients5 15 16 17 but also provides separate estimates for ischemic and hemorrhagic stroke and the proportion of these strokes that may have been attributable to inadequate control of BP. SBP was more strongly associated with the risk of hemorrhagic stroke than DBP, whereas DBP and SBP were similarly associated with the risk of ischemic stroke. Several studies have found a stronger association between hypertension and hemorrhagic stroke than with ischemic stroke.18 19 20 However, several studies in Chinese populations have demonstrated a similar risk of ischemic and hemorrhagic stroke in relation to hypertension.21
Previously it was demonstrated in this same population that 15% of the incident myocardial infarctions among treated hypertensive patients were attributable to uncontrolled BP.22 Possible causes of uncontrolled BP include treatment-resistant hypertension,23 recent start of antihypertensive drug therapy, lack of access to medical care,14 suboptimal treatment by physicians, and patient noncompliance. Lack of compliance with antihypertensive drugs24 25 26 27 28 and lack of appropriate treatment29 30 31 have been identified as the most important barriers to hypertension control. Interventions directed at improvement of compliance32 and optimization of antihypertensive drug treatment33 may lead to improved BP control.
The findings of our study suggest that achieving control of BP among pharmacologically treated hypertensive patients might prevent 57% of hemorrhagic and 27% of ischemic strokes.
| Acknowledgments |
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Received August 18, 1999; revision received November 10, 1999; accepted November 10, 1999.
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